Device and method for extracting a gas hydrate

ABSTRACT

This invention aims to provide a gas-hydrate extracting device and method whereby a high-performance jet fluid is injected from a nozzle at the tip of an extraction pipe inserted into a gas-hydrate stratum, and whereby said jet fluid breaks said stratum so as to form a gas-hydrate mixed fluid that is transferred to surface of the earth, and whereby the void resulting from the removal of said gas hydrate is filled with the components of said high-performance jet fluid and a void-refilling fluid.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a method for extracting fossilfuels, and more particularly to a method for recovering gas from a gashydrate deposited in a formation underground or under the sea floor, andfor preventing the collapse of the formation from which the gas hydratehas been extracted.

PRIOR ART RELATING TO THE INVENTION

[0002] Methane hydrate is deposited in underground sedimentary layersnear the pole regions, hundreds to thousands meters below sea level, asa crystalline structure of methane entrapped or engaged in an expandedlattice of water, and it is regarded as a valuable resource. In order torecover the methane gas from a methane hydrate, it is necessary tochange the temperature, the pressure and the balance of saltconcentration of the hydrate material.

[0003] Several methods have been proposed.

[0004] (1) Heat-stimulation method (Hot water or a hot vapor is pumpedinto a hydrate, which it gasifies.)

[0005] (2) Depressurization method (The pressure of the gas in a hydrateis reduced.)

[0006] (3) Salt-concentration method (Salt water is pumped into ahydrate so as to promote the gasification thereof.)

[0007] (4) Chemical-injection method (Decomposition promoters such asmethanol or glycol are injected into a hydrate so as to promote itsgasification.)

[0008] (5) CO₂-gas (or liquid CO₂) replacing method (Carbon dioxide gas,which is more easily hydrated than methane is, is injected into ahydrate so as to replace the methane.)

[0009] Or, a combination of the above methods can be used.

[0010] Japanese Unexamined Patent Application No. H10-317869 proposed ahigh-pressure vapor-injection method (1) as mentioned above, whichconsists of constructing a gas-shielding wall around the hydrate stratumand then injecting high-temperature vapor to promote the decompositionof the hydrate. Japanese Unexamined Patent Application No. H9-158662proposed the construction of a nuclear reactor at the floor of a deepsea so as to create a flow of warm surface seawater to themethane-hydrate stratum. However, because a void is produced in the seafloor stratum after the methane gas has been extracted, it is fearedthat the above-mentioned methods (1)-(4) can cause some deformation orcollapse of the sea floor, which is fragile.

[0011] Also, in Japanese Unexamined Patent Application No. H6-71161, acarbon-dioxide-gas replacing method has been proposed. In this method,the stratum is replaced with a carbon-dioxide hydrate. However, becausethe CO₂ gas is more easily hydrated than methane is, the injected CO₂gas is sometimes stabilized before the replacement. Therefore, CO₂ gas,although favorable for the purpose of such stabilization, iseconomically unfavorable for the production of methane gas.

[0012] The conventional pressure-reduction method (2) also has a problemin that the possibility of continuous recovery of gas cannot be assuredbecause it greatly depends upon the pressure of free gas, and theconventional chemical-injection method (4) has a problem in that theusage of chemicals is not economical. Furthermore, according to a surveyrelating to methane-hydrate strata at the sea floor, the stratumcontaining methane hydrate is sometimes unstable, and changes such ascollapse and decomposition have occurred repeatedly in the past. Fromthe global point of view, it is necessary on a worldwide level toprevent the dangers of troubles (geohazards) associated with landslides,large-scale sinking or rising of the sea floor, and leakage of naturalgases.

PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] The present invention has been made in view of theabove-mentioned problems, and one object thereof is to provide a methodof extracting a gas hydrate, whereby a gas hydrate is directlytransferred to surface of the earth and the gas is recovered efficientlyby controlling the decomposition of the gas, and whereby the void thatresults after the removal of said gas hydrate is properly filled.

[0014] Another object of the present invention is to provide aneconomical and safe method of extracting a gas hydrate by filling thevoid with industrial by-products from such industrial fields assteelmaking, power generation, and ceramic making. Another object of thepresent invention is to provide a method for preventing the gas-hydratestratum from collapsing after the gas hydrate has been removedtherefrom, which might cause a geohazard.

MEANS FOR SOLVING THE PROBLEMS

[0015] For the purpose of solving the aforementioned problems, thepresent invention's method of extracting a gas hydrate is characterizedsuch that a high-performance jet fluid is injected from a nozzle at thetip of an extraction pipe that has been inserted into a gas-hydratestratum, and said jet fluid breaks the gas-stratum so as to form agas-hydrate mixed fluid that is recovered on the surface of the earth,and the void that results from the removal of the gas hydrate is filledwith the components of said high-performance jet fluid and avoid-refilling fluid

[0016] According to the present invention, the gas hydrate, which isiced or solidified in a gas-hydrate stratum under high pressure and lowtemperature, is broken and is moved to the surface of the earth as agas-hydrate mixed fluid. Therefore, the gas hydrate can be efficientlyextracted from the stratum. In addition, the void resulting from theremoval of the gas hydrate is filled so as to prevent the deformation ofthe ground after the extraction. Therefore, the extraction can becarried out safely. The gas hydrate is also safely recovered from thegas-hydrate stratum, and future geohazards, such as ground subsidence,landslides, or sinking or rising of the sea bottom, can be prevented byfilling the aforementioned void.

[0017] Furthermore, a high-performance jet fluid is used for breakingthe gas-hydrate stratum, so that extraction can be performed withoutloss of power or failure of the mechanism involved, even deeplyunderground or far below the surface of the sea. Also, extraction can besafely performed without adversely affecting the surrounding ground.

[0018] The extraction pipe is inserted near the bottom of thegas-hydrate stratum and is slowly retracted upwardly while rotating.

[0019] According to the present invention, the upward retraction of theinjection nozzle while it is rotating can break the gas hydrate over awide area of the stratum. Therefore, a large volume of a gas-hydratezone can be excavated with a single well (one excavation hole),resulting in improvement of efficiency. If the extraction pipe isinserted further in the horizontal direction at the deep end (bentboring), an even wider area can be covered.

[0020] The void resulting from the removal of the gas hydrate can befilled or replaced with components of the high-performance jet fluid andthe void-refilling fluid. The components are cement, chemicals, andcarbon dioxide gas (CO₂). The stratum can be stabilized by this method.

[0021] In addition, the gas-hydrate mixed fluid is transferred tosurface of the earth as controlled by the injection pressure of thehigh-performance jet fluid, the speed of rotation of the injectionnozzle, and the speed of retraction of the extraction pipe.

[0022] According to the present invention, the breaking or drillingvolume of the gas-hydrate zone can be controlled by the rate of flow ofthe gas-hydrate mixed fluid, which in turn depends on the injectionpressure of the high-performance jet fluid, the speed of rotation of theinjection nozzle, and the speed of retraction of the extraction pipe.

[0023] The gas-hydrate mixed fluid is composed of three phases of air,including gases separated at the gas hydrate zone, water, and the solidsderived from the stratum structure, and the solids are used as thecomponents of the high-performance jet fluid and/or the void-refillingfluid.

[0024] According to the present invention, the area of the gas-hydratezone that is broken can be controlled. Furthermore, the temperature ofthe high-performance jet fluid is higher than that of the gas hydrate,which serves to partially separate the gas and causes an upward flow ofthe gas, which is helpful in minimizing energy consumption. Sedimentsderived from the stratum structure in the gas-hydrate zone are separatedand can be used as the components of the high-performance jet fluidand/or the void-refilling fluid.

[0025] The high-performance jet fluid is composed of air and slurrycontaining fine solids selected from sand and clay.

[0026] According to the present invention, the components of thehigh-performance jet fluid used for breaking the gas-hydrate zone can becommonly used as the void-refilling fluid that is used to fill the voidin the gas-hydrate zone. Air is injected along with the high-performancejet fluid to raise the efficiency of breaking the gas-hydrate stratum.

[0027] The aforementioned fine solids are further selected fromblast-furnace slag, coal ash, and killer.

[0028] According to the present invention, the use of industrialby-products can lower the cost of the void-refilling fluid and, at thesame time, such use provides a means for safely disposing of industrialby-products.

[0029] Preferably the aforementioned fine solids contain at least oneselected from blast-furnace slag, coal ash, and cement.

[0030] According to the present invention, the void resulting from theextraction can be filled and solidified by the use of hardeningmaterials such as cement, blast-furnace slag, coal ash, or killer. Thiscan prevent future landslides and ground subsidence.

[0031] The extraction pipe is a multiple-pipe structure that is composedof (a) a high-pressure pipe by which the high-performance jet fluid isconveyed to the injection nozzle at the tip, (b) a high-performancefluid duct by which the high-performance jet fluid is conveyed to theinjection nozzle at the tip, and (c) a fluid-recovery pipe by which thegas-hydrate mixed fluid is transferred to surface of the earth.

[0032] According to the present invention, the multiple pipe structurecan drill the gas-hydrate zone and transfer the gas-hydrate mixed fluidto the surface of the earth with one boring hole. Therefore, this isapplicable to a gas-hydrate zone even under a deep-sea floor.

[0033] The water of said super high-pressure slurry is river water andspring water from the surface of the earth or seawater from near thesurface of the sea.

[0034] According to the present invention, rich resources such as riverwater, spring water, or seawater can be favorably used, because thelarge temperature difference between the water and the gas-hydrate zoneserves as a heat source for gas decomposition. Gas separation is furtherpromoted by raising the temperature of the water by using sunlight or aheat source.

[0035] The extraction pipe has a control mechanism to control thepressure and speed at which said gas-hydrate mixed fluid is transferredto the surface of the earth.

[0036] According to the present invention, accidents, such as blast jet,that result from rapid gas decomposition can be prevented by controllingthe pressure difference between the gas-hydrate zone and that at thesurface of the earth.

[0037] The present invention's device for extracting a gas hydratecomprises:

[0038] an extraction pipe that is composed of (a) a high-pressure pipeby which the high-performance jet fluid is conveyed to the injectionnozzle at the tip, (b) a high-performance fluid duct by which thevoid-refilling fluid is conveyed to the injection nozzle at the tip, and(c) a fluid-recovery pipe by which the gas-hydrate mixed fluid istransferred to surface of the earth;

[0039] an extraction-pipe control unit that controls the speed ofrotation and the speed of retraction of said extraction pipe;

[0040] an extracting-fluid supply unit that supplies a high-pressurefluid, a void-refilling fluid, and high-pressure air;

[0041] a pressure-control unit of the extraction pipe;

[0042] a gas-extracting device by which gases are recovered from thegas-hydrate mixed fluid;

[0043] Said device is inserted into a boring hole that has been drilledto a gas-hydrate stratum;

[0044] With the gas-hydrate extracting device of the present invention,the aforementioned gas-hydrate extracting method can be realized.

[0045] A high-performance jet fluid is injected so as to break thegas-hydrate stratum, and a void-refilling fluid is injected to fill thestratum so as to compensate for the volume of gas hydrate that has beenremoved.

[0046] According to the present invention, a nozzle of thehigh-performance jet fluid for breaking the gas-hydrate stratum and anozzle of the void-refilling fluid are separately provided, so that bothbreaking and filling can be controlled. This method is realized by themultiple-pipe structure that enables a he high-performance fluid duct tobe inserted into the fluid-recovery pipe.

[0047] The gas hydrate is an ice-like substance including at leastmethane or butane, and said gas-hydrate stratum is a zone in which saidgas hydrate is buried in a state of dispersion, mass, layer, or clusterunder the ground or under the sea floor.

[0048] The process of the present invention can be widely applied to theextraction of any gas hydrate other than a conventional natural-gashydrate. Furthermore, the void of the gas-hydrate stratum that resultsfrom extraction can be filled and stabilized in both land and sea areaswhere troubles (geohazards) might result due to removal of the gashydrate. Therefore, troubles (geohazards) due to deformation of theground can be limited.

EMBODIMENTS OF THE INVENTION

[0049] The embodiment of the present invention will now be described indetail with reference to the drawings.

[0050]FIG. 1(a) shows the structure of the gas-hydrate extracting device100 of the present invention, and FIG. 1(b) shows the structure of thetip end of the extracting pipe 30.

[0051] The gas-hydrate extracting device 100 comprises a platform 101arranged on the sea surface 5 and an extraction pipe 30 inserted into aboring hole 6 drilled near the bottom 1 a of the gas-hydrate stratum 1through the sea-floor stratum 2 a of the sea floor 2. Also, in thisembodiment, the extraction of the gas hydrate from under the sea flooris shown as an example, but in the case of extraction under land, thefacilities on land function similarly as mentioned above.

[0052] Said device further comprises (a) an extraction-pipe control unit10 for regulating the rotation and retraction speeds of the extractionpipe 30, (b) an extraction-fluid supply unit 20 for supplyinghigh-pressure fluids containing a void-refilling fluid 21 andhigh-pressure air, (c) an extraction-pipe pressure-control unit 15 thatcontrols the pressure of said extraction pipe 30, and (d) agas-extracting device 25 for recovering gas from the gas-hydrate mixedfluid 4, which contains some sediments from the gas-hydrate-stratumstructure.

[0053] As shown in FIG. 1(b), said extraction pipe 30 has a triplestructure, wherein are arranged (a) a fluid-recovery pipe 31, (b) ahigh-pressure pipe 33 having an injection nozzle 33 a for thehigh-performance jet fluid 3, and (c) a high-performance fluid duct 32having an injection nozzle 32 a for the void-refilling fluid 21.

[0054] The illustrated embodiment shows a condition such that ahigh-pressure pipe 33 is inserted into a high-performance fluid duct 32,but the fluid duct 32 can be inserted into the high-pressure pipe 33.Also, the high-pressure pipe 33 can have a structure such that theslurry and the high-pressure air are conveyed separately and are joinedat the injection nozzle 33 a (not shown). The structure of theextraction pipe 30 is not limited to this embodiment, but should beselected according to the conditions of the extracting site.

[0055] The high-performance fluid duct 32 injects, by rotating, thehigh-performance jet fluid 3 and the void-refilling fluid 21 into thesurrounding gas-hydrate stratum 1, so as to break up that stratum. Theresulting gas-hydrate mixed fluid 4 is transferred through thefluid-recovery pipe 31. At that time, the high-performance jet fluid 3and the void-refilling fluid 21 are inserted into the stratum so as tocompensate for the volume of gas hydrate that has been removed.

[0056] The extraction-pipe control unit 10 controls the extraction pipe30 so as to insert it near the bottom of the gas-hydrate stratum and toretract it back to the surface of the earth while rotating the injectionnozzles 32 a, 33 a at the tip of the extraction pipe 30 and whileinjecting the high-performance jet fluid 3 and the void-refilling fluid21 into the stratum surrounding the gas hydrate. At this time, thegas-hydrate mixed fluid 4 is transferred to the surface of the earth,and the void resulting from the removal of the gas hydrate is filledwith the solid components of the high-performance jet fluid 3 and thevoid-refilling fluid 21.

[0057] The extraction pipe 30 can drill through the sea floor and beinserted into the gas-hydrate zone 1 using a drilling device (such as aboring bit) at the tip of the extraction pipe 30.

[0058] At this time, the extracting-fluid supply unit 20 controls thebreaking area in the gas-hydrate zone 1 by adjusting the injectionpressure of the high-performance jet fluid 3 and the void-refillingfluid 21. The extraction-pipe control unit 10, by adjusting the speed ofrotation of the high-performance fluid duct 32 and the speed ofretraction of the extraction pipe 30, controls the speed at which thegas-hydrate mixed fluid 4 is extracted.

[0059] At the top of said mixed-fluid-recovery pipe 31 is anextraction-pipe pressure-control unit 15, which is a pressure-controlmechanism that controls the pressure of the fluid-recovery pipe 31 sothat the pressure of the gas-contained mixture fluid 4 to be transferredto the surface of the earth is controlled so that the gasification ofthe cut and broken gas hydrate is controlled and the recovery speed ofthe gas-contained mixture fluid 4 containing stratum slime also iscontrolled.

[0060] The gas-extracting device 25 separates and recovers gas from thegas-hydrate mixed fluid 4. The gas-hydrate mixed fluid 4 that istransferred to surface of the earth or to a sea platform is composed ofthree phases of air including the gas separated from the gas-hydratestratum, water, and solids from the stratum structure.

[0061] The gas-extracting device 25 supplies to the extracting-fluidsupply unit 20 the solid residue that remains after gas separation andthat is to be used as a component of the high-performance jet fluid 3and/or the void-refilling fluid 21.

[0062]FIG. 3 is a schematic diagram illustrating the scheme for reusingthe recovered gas-hydrate mixed fluid. The gas-extracting device 25separates gas and solid residues from the gas-hydrate mixed fluid 4,conveys the separated gas to gas-storage/transportation units (notshown), and conveys the solid residue to the extracting-fluid supplyunit 20.

[0063] In the extracting-fluid supply unit 20, solids selected from finesand, clay, and fine granular materials including industrial by-productssuch as cement, blast-furnace slag, and coal ash, are incorporated inboth the high-performance jet fluid and the void-refilling fluid. Theuse of industrial by-products can lower the cost of the fluids andachieve safe disposal of such by-products without causing any pollution.

[0064] If a sea platform is used, seawater near the surface of the seais preferably used, because, due to the high temperature of the seawaterand due to its nature as saltwater, the heat balance and thesalt-concentration balance of the gas hydrate can be made to vary so asto promote gas separation. When further gas separation is required, thewater temperature should be raised by a heat source, which could besunlight. Where on-land facilities are used, usually river water orspring water is used.

[0065]FIG. 2 shows a schematic diagram in another embodiment (bentboring) of gas-hydrate extraction.

[0066] In this embodiment, the boring is performed horizontally in thegas-hydrate stratum 1 as a bent boring hole 6 a. The extraction pipe 30is inserted into the deep end 1 b of the stratum. Then, a bent boringhole 6 bis similarly drilled into the gas-hydrate zone 1, and a bentboring hole 6 c is drilled into the gas-hydrate zone 1 as well. In thisembodiment, even with a single well (one drilling hole), gas-hydrate canbe extracted from a wider area of the gas-hydrate zone. This methodimproves extraction efficiency.

[0067] Because gas hydrates exist in natural environments that are in astate of delicate balance, there is always the danger that a collapse ordeformation of the ground will occur due to some external factor such asan earthquake or that leakage of gas will result due to a landslide orsinking or rising of the ground. The present invention can be applied toa method for recovering gas from the gas hydrate located in an unstableland or sea area, and for stabilizing the stratum after the gas isextracted.

[0068]FIG. 4 is a schematic diagram illustrating an operation ofgas-hydrate extraction. In the placing and preparing step (1) a platform(of a drilling ship) equipped with a gas-hydrate extracting device movesover the sea surface 5 in the area where the gas-hydrate stratum islocated under the sea floor.

[0069] In boring step (2), a boring hole 6 is drilled such that itpenetrates through the sea-bottom stratum 2 a and reaches the bottom-endlayer 1 a of the gas hydrate layer 1.

[0070] In the gas extraction/replacement start step (3), an extractionpipe 30 is inserted into the boring hole 6, and the high-performancefluid duct 32 is rotated so as to inject the high-performance jet fluid3, so that the surrounding gas-hydrate stratum 1 is broken.

[0071] In the extraction-pipe retraction step (4), the extraction pipe30 is retracted, injecting the high-performance jet fluid 3 and thevoid-refilling fluid 21 so as to break the surrounding gas-hydratestratum 1 and to fill the resulting void with the fluids. The retractionof the extraction pipe 30 makes the extraction area wider towards thetop of the stratum of the gas hydrate layer 1.

[0072] In the replacement-completion step (5), the injection is stoppedwhen the retraction of the injection point reaches the top of thegas-hydrate stratum 1.

[0073] In the extraction-pipe removal step (6), the extraction pipe 30is completely retracted to the surface of the earth and is moved to thenext drilling site.

[0074] As to the gas-hydrate extracting device 100 in this example, oneextraction pipe 30 and one extraction-pipe control unit 10 are used, butplural extraction pipes 30 can be simultaneously controlled from theplatform 101.

[0075] The injection pressure is typically 150 Mpa or more for theextraction of the gas hydrate at a distance of as far as 8 meters aroundthe extraction pipe 30, though the pressure should be decided upon basedon the conditions of the gas hydrate and the depth of the stratum,

[0076] If the gas-hydrate zone 1 is composed 20% of methane hydrate, andif the methane hydrate is composed 80% of methane, the volume of themethane hydrate becomes 216 times greater when the gas-hydrate isgasified. One cubic meter of the gas hydrate produces 35 cubic meters ofmethane. When extraction is performed at a retraction speed of 10m/hour, 400,000 cubic meters of methane gas can be extracted in one day

EFFECTS OF THE INVENTION

[0077] The device and method for extracting gas hydrate of the presentinvention provide the following benefits.

[0078] According to the present invention, a gas hydrate, which is icedor solidified in a gas-hydrate stratum under high pressure and lowtemperature, is broken and then transferred to surface of the earth as agas-hydrate mixed fluid. Therefore, a gas hydrate can be efficientlyextracted from the stratum. In addition, the void resulting in thestratum due to the removal of the gas hydrate is filled so as to preventthe deformation of the ground after the extraction. Therefore, theextraction can be carried out safely.

[0079] Furthermore, a high-performance jet fluid is used for breakingthe gas-hydrate stratum, so that extraction can be performed with littleloss of power and without failure of the mechanism used, even deeplyunderground or under the ground beneath the sea. Also, extraction can besafely performed without adversely affecting the surrounding ground.

[0080] According to the present invention, the retraction of theinjection nozzle while it is being rotated can cover a wide area of thestratum so as to break the gas hydrate. Therefore, a large volume of agas-hydrate zone can be excavated with a single well (one excavationhole), resulting in improved extraction efficiency. If the extractionpipe is inserted further in the horizontal direction at the deep end(bent boring), an even wider area of the gas-hydrate zone can becovered. The void resulting from the removal of the gas hydrate can befilled or replaced with slurry composed of the components of thehigh-performance jet fluid and a void-refilling fluid. The componentsare cement, chemicals, and carbon dioxide gas (CO₂). The stratum can bestabilized by this method.

[0081] According to the present invention, the breaking area and thedrilling volume of the gas-hydrate layer can be controlled. Furthermore,the high-performance jet fluid, which has a higher temperature than thegas hydrate, partially separates the gases of the gas-hydrate layer andforms an upward flow with the gas, which minimizes energy consumption.Sediments derived from the stratum structure of the gas-hydrate zone areseparated and can be used as the component of the high-performance jetfluid and/or the void-refilling fluid use to fill the void that resultsfrom the extraction.

[0082] According to the present invention, the composition of thehigh-performance jet fluid used for breaking the gas-hydrate zone can beused as a void-refilling fluid to fill the void resulting from theextraction. Air is injected along with the high-performance jet fluid soas to raise the efficiency of breaking the gas-hydrate stratum. The useof industrial by-products can lower the cost of the void-refilling fluidand, at the same time, can provide a means for safely disposing of thoseindustrial by-products. The refilled void can be solidified by the useof hardening materials such as cement, blast-furnace slag, coal ash, andkiller. Such solidification can prevent landslides.

[0083] According to the present invention, a multiple-pipe structure canbe used to drill the gas-hydrate zone and to transfer the gas-hydratemixed fluid to the surface of the earth with only one boring hole.Therefore, a gas hydrate can be efficiently extracted even from agas-hydrate zone that is under a deep-sea floor.

[0084] According to the present invention, rich resources such as riverwater, spring water, or seawater can be favorably used, because thelarge temperature difference between the water and the gas-hydrate zoneserves as a heat source for gas decomposition. Gas separation can befurther promoted by increasing the temperature of the water by usingsunlight or another heat source.

[0085] According to the present invention, rapid gas decomposition suchas blast jet can be prevented by controlling the difference in pressurebetween that of the gas-hydrate zone and that at the surface of theearth.

[0086] According to the present invention, the process can be widelyapplied to the extraction of any gas hydrate other than a conventionalnatural-gas hydrate. Furthermore, the void in the gas-hydrate stratumthat results from extraction can be filled and stabilized in bothunder-land and under-sea areas, where troubles (geohazards) due toremoval of the gas hydrate might result. Therefore, troubles(geohazards) due to deformation of the ground can be limited.

BRIEF DESCRIPTION OF THE DRAWINGS

[0087]FIG. 1(a) shows the structure of a gas-hydrate extracting device100 of the present invention, and FIG. 1(b) shows the structure of thetip end of an extracting pipe 30.

[0088]FIG. 2 is a schematic diagram of the gas-hydrate extractionprocess in another embodiment (bent boring).

[0089]FIG. 3 is a schematic diagram illustrating a scheme for reusingthe recovered gas-hydrate mixed fluid.

[0090]FIG. 4 is a schematic diagram illustrating an operation procedureof gas-hydrate extraction.

EXPLANATION OF THE NUMERALS AND SYMBOLS IN THE DESCRIPTION AND THEDRAWINGS

[0091]1. gas-hydrate zone

[0092]2. sea floor

[0093]2 a stratum below sea floor

[0094]3 high-performance jet fluid

[0095]4 gas-hydrate mixed fluid

[0096]5 sea surface

[0097]6 boring hole

[0098]6 a, 6 b, 6 c bent boring holes

[0099]10 extraction-pipe control unit

[0100]15 extraction-pipe pressure-control unit

[0101]20 extracting-fluid supply unit

[0102]21 void-refilling fluid

[0103]25 gas-extracting device

[0104]30 extraction pipe

[0105]31 mixed-fluid-recovery pipe

[0106]32 high-performance fluid duct

[0107]32 a injection nozzle

[0108]33 high-pressure pipe

[0109]33 a high-performance fluid-injection nozzle

[0110]100 gas-hydrate extracting device

[0111]101 platform

What is claimed is:
 1. A gas-hydrate extracting method wherein (a) ahigh-performance jet fluid is injected from a nozzle at the tip of anextraction pipe that has been inserted into a gas-hydrate stratum, (b)said jet fluid breaks said stratum so as to form a gas-hydrate mixedfluid that is transferred to surface of the earth, and (c) the voidresulting from the removal of said gas hydrate is filled with thecomponents of said high-performance jet fluid and a void-refillingfluid.
 2. A gas-hydrate extracting method as described in claim 1,wherein said extraction pipe is inserted to the bottom of saidgas-hydrate stratum and is slowly retracted upward while being rotated.3. A gas-hydrate extracting method as described in claim 1 or 2, whereinsaid gas-hydrate mixed fluid is transferred to surface of the earth at arate controlled by the injection pressure of said high-performance jetfluid, the rotation speed of said injection nozzle, and the speed atwhich said extraction pipe is retracted upward.
 4. A gas-hydrateextracting method as described in any one of claims 1-3, wherein saidgas-hydrate mixed fluid is composed of three phases—air containing gasesseparated at the gas hydrate zone, water, and solids derived from thestratum structure—and with said solids being used as the components ofsaid high-performance jet fluid and/or said void-refilling fluid.
 5. Agas-hydrate extracting method as described in any one of claims 1-4,wherein said high-performance jet fluid is composed of air and asuper-high-pressure slurry formed by mixing water, fine sand, andviscous clay.
 6. A gas-hydrate extracting method as described in claim5, wherein said super high-pressure slurry is composed of fine granularmaterials including industrial by-products such as blast-furnace slag,coal ash, and killer instead of said fine sand and viscous clay.
 7. Agas-hydrate extracting method as described in any one of claims 5 and 6,wherein said fine granular materials include at least one solid selectedfrom blast-furnace slag, coal ash, and cement.
 8. A gas-hydrateextracting method as described in claim 1, 2, or 3, wherein saidextraction pipe has a multiple-pipe structure that is composed of (a) ahigh-pressure pipe by which said high-performance jet fluid is conveyedto the injection nozzle at the tip of said extraction pipe, (b) ahigh-performance fluid duct by which said high-performance jet fluid isconveyed to the injection nozzle at the tip, and (c) a fluid-recoverypipe by which said gas-hydrate mixed fluid is transferred to surface ofthe earth.
 9. A gas-hydrate extracting method as described in any one ofclaims 5-7, wherein river water and/or spring water on the surface ofthe ground, or seawater near the surface of the sea, is used as thewater of said super-high-pressure slurry.
 10. A gas-hydrate extractingmethod as described in claim 8, wherein said extraction pipe has acontrol mechanism to control the pressure and the transfer speed of saidgas-hydrate mixed fluid.
 11. A gas-hydrate extracting device comprising:an extraction pipe that is composed of (a) a high-pressure pipe by whichhigh-performance jet fluid is conveyed to an injection nozzle at the tipof said extraction pipe, (b) a high-performance fluid duct by which avoid-refilling fluid is conveyed to the injection nozzle at said tip ofsaid extraction pipe, and (c) a fluid-recovery pipe by which agas-hydrate mixed fluid is transferred to surface of the earth, withsaid extraction pipe being inserted into a boring hole drilled into agas-hydrate stratum; an extraction-pipe control unit that controls therotation speed and the speed of retraction of said extraction pipe; anextracting-fluid supply unit that supplies a high-pressure fluid, avoid-refilling fluid, and high-pressure air; a pressure-control unit ofsaid extraction pipe; a gas-extracting device by which gases arerecovered from said gas-hydrate mixed fluid.
 12. A gas-hydrateextracting method as described in claim 11, wherein saidhigh-performance jet fluid is injected into said gas-hydrate stratum soas to break said stratum, and wherein said void-refilling fluid isinjected so as to compensate for the volume of said gas hydrate that hasbeen removed from the stratum.
 13. A gas-hydrate extracting method asdescribed in any one of claims 1-12, wherein said gas hydrate is anice-like substance including at least methane or butane, and whereinsaid gas-hydrate stratum is a zone in which said gas hydrate is buriedin a state that constitutes a dispersion, a mass, a layer or a clusterunder the ground or under the sea floor.